The present invention relates to plasma reactors, and more particularly, to plasma reactors which uniformly condition workpieces treated by a gas discharge plasma within said reactor.
In the manufacture of multilayer printed circuit boards (either of the rigid or flexible variety), interconnecting holes are drilled through the boards and interconnecting metallic layers are plated within the drilled holes to provide an electrical connection between exposed edge portions of the conducting layers of the printed circuit boards. Typically, the printed circuit board's conducting layers are defined patterns of copper, separated by layers of insulating plastic.
A problem which has been encountered in forming interconnecting holes is commonly known as "drill smear" in the printed circuit board art. The drill smear problem is the result of resin from the board, as well as bonding agents that hold the boards together, coating the inside surface of the interconnecting holes. The resulting smeared layers tend to insulate the edge portions of the conducting layers exposed within the drilled holes, and if not removed prior to plating of the apertures, individual circuits will be shielded from the plating and, therefore, not function properly.
In the past the problem of drill smear was treated by either wet (acid) chemistry or dry (plasma) chemistry; however, each of these methods was plagued with numerous problems. A typical example of using the wet chemistry method to attack drill smear can be found in U.S. Pat. No. 4,155,775 to Alpaugh et al. Likewise, an example of utilizing dry chemistry to solve this problem can be found in U.S. Pat. No. 4,012,307 to Phillips.
In utilizing wet chemistry, corrosive chemicals are used to attack the smear and transform it into a residue that is then washed away with water, whereas in dry chemistry, a plasma is used to chemically convert the drill smear into gaseous by-products that are carried away by a mechanical pump.
In general, wet chemistry is considered the less desirable method since it creates undue hazards for personnel and excess pollutants both in the form of vapor and waste materials that are difficult to dispose of properly. Moreover, plasma de-smearing is a one-step operation as compared to the wet de-smearing operation which is multistep. Also, the dry chemistry method etches back the non-metallic portion of the multilayer printed circuit board adjacent to the conducting layers in the region of the drilled holes, thereby providing an increased exposed surface area of the conducting layers to which the interconnecting metallic layer is subsequently plated. Accordingly, improved mechanical adhesion of the interconnecting metallic layer results from the etching back operation.
While it is advantageous to use dry chemistry, certain difficulties and deficiencies have been encountered in prior art plasma reactors. In particular, the electrode and chamber design utilized in such plasma reactors are not compatible to large scale production systems. Furthermore, non-uniformity in workpiece conditioning has been encountered when utilizing large scale plasma reactor systems having annular-shaped electrodes.
Therefore, it is an object of the apparatus and method of the present invention to overcome the heretofore described deficiencies of the prior art.
A particular objective of the present invention is to provide uniformly conditioned workpieces treated in a plasma reactor apparatus suitable for large scale production operations.
These and other features and attendant advantages of the present invention will be more fully appreciated as the same become better understood from the following detailed description thereof.